This invention relates generally to radiotelephones and, in particular, to wireless terminals or mobile stations that are capable of operation with a wireless network having packet data capability.
In a GPRS-136 packet data mode of operation (and possibly in others) a wireless terminal may not know if a particular received time slot (referred to herein as a downlink time slot) is directed to the wireless terminal or to some other wireless terminal. As such, the downlink time slot must be completely received and the data detected in order for the wireless terminal to determine, from information relating to the upper protocol layer(s), the intended recipient for the slot.
The same is true even for the case where the transmitting base station does not have data to transmit to the wireless terminals served by the base station, in which case the base station may simply transmit random data that is not addressed to any particular wireless terminal. However, unless the wireless terminals actually receive and decode such a slot, they will not know if the slot contains valid data that is addressed to them.
As can be appreciated, the requirement to completely receive and then decode the downlink time slots can have a detrimental impact on the battery life of the wireless terminal.
FIGS. 1, 2 and 3 illustrate a conventional TDMA frame format, a down link packet data channel (PDCH) slot structure when using xcfx80/4-DQPSK modulation, and the downlink PDCH slot structure using 8-PSK modulation, respectively. Each slot is assumed to have a duration of about 6.67 milliseconds. The CSFP/CDFT is used to convey a five bit Superframe Phase (SFP) value and a three bit Data Frame Type (DFT) value. The 14 symbol synchronization (SYNC) field, six symbol CSFP/CDFT field, and 12 symbol Packet Control Field (PCF) together form a 32 symbol slot Header portion, from which the recipient of the slot can be ascertained by a receiver. The 32 symbol Header portion is followed by a 130 symbol data portion. In this embodiment the 32 symbol Header portion is always modulated using xcfx80/4-DQPSK, while the remaining portion of the slot can be modulated using either xcfx80/4-DQPSK or 8-PSK.
It is known that in order to reduce interference a base station can lower the transmitted power of the slot following the Header portion, for the case where the base station has no data to transmit to wireless terminals.
More particularly, a Quasi-Discontinuous Transmission (Q-DTX) is a form of downlink power control that allows the base station to reduce its output power during a portion of each downlink time slot in order to reduce the interference in the system. Reference can be had to TIA/EIA-136-331 for the cases where Q-DTX based power reduction may be applied to the downlink of a PDCH. For the cases where Q-DTX based power reduction may be applied the base station may only invoke it for those PDCH downlink slots where it has nothing to send.
The slot Header portion of a downlink time slot is not subject to Q-DTX. When Q-DTX based power reduction is allowed it may therefore only be applied to the portion of a downlink slot following the slot Header portion.
However, while this technique may reduce system interference and may also lower the base station""s power consumption (which is normally not as an important a consideration as reducing the wireless terminal""s power consumption), it will have no effect on reducing the power consumption of the wireless terminals that receive a PDCH from the base station.
It is thus a first object and advantage of this invention to provide an improved method for reducing the power consumption of a wireless terminal that receives packet data from a base station.
It is another object and advantage of this invention to employ the Quasi-Discontinuous Transmission (Q-DTX) mode of operation of the base station so as to detect when the base station is not transmitting valid packet data, so as to reduce the power consumption of the wireless terminal.
It is a further object and advantage of this invention to employ the Quasi-Discontinuous Transmission (Q-DTX) mode of operation of the base station so as to terminate the reception and decoding of a particular downlink time slot prior to the end of the time slot, when the base station is not transmitting valid packet data, thereby enabling a reduction in the power consumption of the wireless terminal.
The foregoing and other problems are overcome and the objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.
A method is disclosed for operating a wireless terminal in a wireless communication system that operates with frames time divided into slots each having a Header portion followed by a Data portion. The system is arranged to transmit a downlink slot so that the Header portion is transmitted at a higher power level than the Data portion when the Data portion does not contain valid data so as to reduce system interference. This is known as a Quasi-Discontinuous Transmission (Q-DTX) mode of operation. The method includes steps of (A) receiving all of the Header portion and only a part of the Data portion of a slot and detecting whether the Header portion was transmitted at a higher power level than the Data portion is being transmitted; and (B) if it is detected that the Header portion was transmitted at a higher power level than the Data portion is being transmitted, terminating the reception of a remaining part of the slot and placing at least a portion of the wireless terminal in a reduced power consumption state. Otherwise, if it is detected that the Header portion was transmitted at the same power level as the Data portion is being transmitted, continuing to receive the remaining part of the slot.
More particularly, the method includes a first step of performing a channel estimation in a forward direction over at least some symbols of a received Header portion to derive a first channel estimate (EST1) at the end of the Header portion; and a second step of performing a channel estimation in a reverse direction over a subset of symbols of the Data portion so as to derive a second channel estimate (EST2) at the beginning of the Data portion. A next step obtains a ratio of the derived channel estimates (EST1/EST2), and a further step compares the ratio to a threshold value to determine if the Data portion contains valid data.
If the Data portion is determined not to contain valid data, the method places at least a receiver of the wireless terminal in a reduced power consumption state for a remainder of at least the current slot, while if the ratio is determined to be about unity the Data portion is assumed to contain valid data, and a receiver of the wireless terminal is left in a fully powered, operational state to receive the remainder of the current slot.
The steps of estimating may use a Least Mean Squares (LMS) channel estimator, although other channel estimation techniques could be used as well. In one embodiment the threshold value is fixed during the operation of the wireless terminal, while in another embodiment the threshold value is made variable during the operation of the wireless terminal so as to have a value that is a function of at least one influencing factor. The at least one influencing factor can be selected from at least one of a velocity of the wireless terminal or an amount of noise and/or interference in the channel.
In a further embodiment the method the method first makes a determination as to whether a last received N slots contained valid packet data for the wireless terminal. If the determination is in the affirmative, the wireless terminal receives an entire next slot, as it is assumed that the probability is increased that the next slot will contain valid packet data for the wireless terminal. Otherwise, if the determination is in the negative, the method begins to receive the next slot and performs the above described channel estimation in a forward direction, followed by the other steps.